Syringe assemblies having a needle (“cannula”) are commonly used for the delivery of fluids, e.g. medicaments, into patients, and/or for aspirating fluids from the patients. Desirably, these syringe assemblies can be operated by a single hand, so that a doctor or nurse is able to operate them while using his or her other hand for another purpose. The force required should not be very great, or the doctor or nurse's hand will become tired if many injections are performed. Furthermore, it is often undesirable for the same needle of the syringe assembly to be used for multiple patients, and several forms of syringe assemblies are known which prevent or discourage this.
A first form of syringe assembly comprises a disposable needle assembly consisting of a needle and a connector element. Using the connector element, the needle assembly is temporarily attached to a forward end of an elongate syringe unit, to form the syringe assembly. The syringe unit has a barrel, and a plunger movable forward within the barrel towards the needle assembly. A medicament fluid is located in a volume between a front face of the plunger and the needle assembly. The volume communicates with the inside of the needle, so that as the barrel is advanced the fluid is driven through the needle. When this happens, it is undesirable to have fluid leaking through the junction where the circumference of the front face of the plunger meets the walls of the barrel. In order to be compliant with the ISO 7886-1 standard, the syringe unit has to withstand a compression force resulting in an internal test pressure of 300 kPa (3 bars) without leaking. When the medicament has been dispensed, the needle assembly is removed and disposed of.
Examples of such syringe assemblies are the Luer Taper and Luer Lock designs standardized under ISO 594. The Luer Taper variant uses a press fit connection between the needle assembly and syringe unit that works using friction. Unfortunately, the needle assemblies are prone to loosening and dropping off the syringe unit. The Luer Lock variant seeks to resolve the problem of loosening associated with the Luer Taper by using a thread locking system. The Luer Lock however has a complicated design involving multiply nested interlocking portions. Specifically, the syringe unit has a conical spout encircled by an internally threaded collar. The connection element of the needle assembly carries an external thread for engaging the collar, and defines a cavity for receiving the spout. The connection element is inserted into the gap between the spout and the collar.
Both these syringe assemblies contain a large amount of dead space—i.e. volumes in the syringe assembly which contain medicament which cannot be expelled from the syringe assembly. There is typically dead space within the connector unit and/or the interface between the connector unit and syringe unit. This dead space is an issue of concern as the cost of medicament may be high. It is possible to reduce the size of the dead space by reducing the tolerances in the production process, but this increases the production cost of the syringe assembly.
Another known form of syringe assembly is a “retractable syringe”, which comprises a syringe unit, a needle unit (which in some forms of the retractable syringe is just a needle, but in others is a needle and a “needle hub”, i.e. an encircling body of material, typically molded plastics material) located at one end of the syringe unit, a retention mechanism for maintaining the position of the needle unit with respect to the syringe unit, and a drive mechanism. Operation of syringe causes the retention mechanism to be disabled after the syringe assembly has been used to deliver a medicament, and the drive mechanism then drives the needle unit into the syringe unit, so that the needle is no longer exposed. Retractable syringes are illustrated in U.S. Pat. No. 6,994,690, WO 2005/053779, U.S. Pat. No. 6,494,863, US 2008/0033355 and U.S. Pat. No. 7,351,224. Another retractable syringe is the BD Integra Syringe marketed by Becton, Dickinson and Company of New Jersey, United States. This retractable syringe has a detachable needle, and uses a form of connection between the syringe unit and the needle reminiscent of the Luer Lock system. Experiments have found that it may take an “activation” force of about 55N in order to break the retention mechanism of a 1 ml version of the Integra syringe. This force is estimated to be over 5 times the compression force which the syringe unit has to withstand without leaking in order to achieve the ISO 7886-1 standard.
Some of the earlier retractable syringes have thus reduced the amount of “activation” force required by having more dead space within the syringe and/or by staggering the activation process. In cases where staggering is used, dead space may be required to provide room for the cascaded movement of the individual elements involved in the activation process. Therefore, most designs of retractable syringe have dead spaces, resulting in wastage of the medicament. In some designs air may be trapped in the dead space, and is hard to expel this air before the syringe assembly is used.
Furthermore, since the retention mechanism has to be strong enough to resist the urging force generated by the drive mechanism, the user has to apply an inconveniently large force to break the retention mechanism, and release the needle unit.
Furthermore, the needle unit, syringe unit, retention mechanism and drive mechanisms of many known retractable syringes have too many parts, leading to high production cost. Since retractable syringes are expensive, some operatives are tempted to try to re-use them, and there is little to prevent this since almost all components remain intact following operation of the device.